Add optional leading whitespace highlighting capability

[AgentHagu]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Fixes #2573

Overview of changes:
Allows users to now optionally highlight leading whitespace, giving them more options when using MarkBind.

Users can now add an optional + after their highlight index specifier to indicate that the index should be inclusive of leading whitespace. For example previously 1[:4] would highlight the abcd in abcd. But now, by instead using 1[:4]+, MarkBind will highlight ab in abcd (i.e. it will highlight the first 2 whitespace in the string).

Anything you'd like to highlight/discuss:

Testing instructions:

Proposed commit message: (wrap lines at 72 characters)
Add optional leading whitespace highlighting capability

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).